Catalytic converters are muffler-like devices which reduce the amount of harmful components in the exhaust gas of an internal combustion engine. The catalytic converter contains a catalyst which promotes chemical reactions, converting exhaust gas components such as CO, HC and NO.sub.x into CO.sub.2, H.sub.2 O, N and O.sub.2.
The efficiency of a catalytic converter dramatically improves as the temperature of its catalyst increases. Consequently, when a cold engine is started, the exhaust gases discharged into the atmosphere contain a higher percentage of CO, HC and NO.sub.x until the catalyst heats up. The catalyst is normally heated by the temperature of the exhaust gases passing through the converter, taking several minutes after engine start-up to achieve optimum temperature.
Several attempts have been made to reduce the amount of time required for the catalyst to heat up. These attempts have principally consisted of applying an external source of heat to the catalytic converter. One such method comprises a preheater which ignites an air/fuel mixture at a point slightly upstream of the catalytic converter whereby the released combustion energy operates to rapidly increase the catalyst temperature. Ignition is provided for by a periodic spark in proximity to the air/fuel mixture, such as from a conventional spark plug driven by a periodic energization signal from a controller.
With such systems, failure to ignite the air/fuel mixture will result in a substantial quantity of unburnt fuel being passed directly into the atmosphere, increasing the levels of undesirable emissions. Therefore, there is a diagnostic desire to determine whether fuel combustion has taken place. If the air/fuel mixture fails to ignite, the fuel supply and energization signal can be interrupted.